Conventionally, it has been thought that mercury exists in exhaust gas in forms of metallic mercury insoluble in water and mercury chloride soluble in water. If metallic mercury can be converted to mercury chloride in the presence of a catalyst such as a denitrification catalyst, mercury can be removed by a desulfurization apparatus on the downstream side (for example, see Japanese Patent Provisional Publication No. 10-230137 (No. 230137/1998)).
However, when mercury is converted to mercury chloride by using the denitrification catalyst, the conversion temperature is limited to 300 to 450°, so that in the case of coal with a low Cl content, it is necessary to supply HCl into exhaust gas to maintain the rate of conversion to mercury chloride, which presents problems of increased utility cost and occurrence of material corrosion.
On the other hand, a method can be thought of in which a mercury oxidation catalyst is provided separately from the denitrification catalyst so that the conversion can be accomplished at a low temperature, and thereby metallic mercury is converted to mercury chloride in the presence of mercury oxidation catalyst.
However, when metallic mercury is converted to mercury chloride by using a mercury oxidation catalyst at a low temperature, excess NH3 having been injected in an upstream denitrification apparatus flows down and adheres onto the catalyst as ammonium sulfate or acid ammonium sulfate, by which the catalyst performance is sometimes deteriorated greatly. Also, there arises a problem in that the catalyst itself is closed by the adherend. Specifically, in an ammonia denitrification apparatus applied to a combustion boiler, ammonia (NH3) is injected through an injection nozzle in the combustion boiler or on the downstream side thereof to be allowed to react reductively with nitrogen oxides (NO, NO2), by which a process for decomposing nitrogen oxides into nitrogen (N2) and water (H2O) is carried out. Ammonia added excessively in this process exists in the exhaust gas and flows down to the downstream side of the denitrification catalyst, and thereby produces ammonium sulfate or acid ammonium sulfate.
On the other hand, even when either of the denitrification or mercury oxidation catalysts is used, the coexistence of NH3 hinders a mercury oxidation reaction and hence causes an increase in the filling amount of catalyst.